1. Field of the Invention
The present invention relates to a communications navigation system in which transmission and reception of data concerning position information and route/guidance information necessary for route guidance is carried out using communication between a navigation apparatus as a terminal which is mounted on a moving body such as a vehicle or formed into a portable mobile terminal and a navigation base apparatus acting as a base, thereby providing route information and guidance information (hereinafter, referred to as "route/guidance information" or "route/guidance data") to the navigation apparatus to carry out route guidance.
2. Description of the Prior Art
In the navigation systems commonly being used, a navigation apparatus is mounted on a moving body such as a vehicle, in which route guidance is carried out by utilizing map data stored in a recording medium such as CD-ROM provided in the respective vehicles. However, such systems require the user thereof to sometime buy new CD-ROMs in order to obtain new information concerning the construction of new roads and the abolition of old roads and the like. Further, when the type of recording medium is changed (e.g., to DVD-ROM) or when a different format is used with the same type of recording medium, the navigation apparatus must be replaced with other one that is suited for such new type recording medium or format.
In view of such problems, Japanese Laid-Open Patent Application No. HEI 10-19588 discloses a communications navigation system in which map images and recommended route data necessary for guiding a vehicle to a destination are transmitted from a navigation base (center) to a vehicle such as an automobile.
In more details, in this system, communication is carried out between a route/guidance data transmission system at the navigation base and a vehicle navigation apparatus mounted on the vehicle. The route/guidance data transmission system includes a database which stores data necessary for guiding a vehicle to a destination. In response to a request from the navigation apparatus, the route/guidance data transmission system carries out a route search to search a recommended route, and then reads out necessary route/guidance data from the database. After these operations are completed, the route/guidance data transmission system transmits the route/guidance data of the recommended route to the navigation apparatus. Then, the navigation apparatus displays guidance information based on the route/guidance data of the recommended route transmitted from the route/guidance data transmission system.
In the above-described conventional communications navigation system, all of the route/guidance data from the departure point to the destination produced at the navigation base is transmitted to the navigation apparatus as they are. In the vehicle, route/guidance information corresponding to the present position of the vehicle is successively displayed on the display of the navigation apparatus all over the route from the departure point to the destination.
However, the above-described communications navigation system involves the following problems.
(1) First, as described above, in the conventional communications navigation system, all of the route/guidance data from the departure point to the destination are transmitted to the vehicle. Therefore, the amount of the data to be transmitted becomes huge, so that it takes a long time to transmit the data and further it is necessary for the navigation apparatus to temporally store the huge amount of the transmitted data. Further, such a long data transmission time not only increases communication cost at the user but also lengthens the time required for processing the data. In particular, when the distance between the departure point and the destination is long, such increases in the communication time and communication cost reach a degree that can not be disregarded. Furthermore, such long communication time gives rise to communication rush, which in turn leads to a problem in that communication errors or communication interrupts are likely to occur.
Further, it seems that in many cases users recognize traveled roads on which the users have traveled in the past. In such cases, the users will be able to reach the destination without detailed route guidance if users can have route/guidance information only for untraveled portions in the route to the destination. This means that transmission of the route/guidance information for such traveled roads leads to increase in the amount of data to be transmitted.
(2) In the conventional navigation system, the route/guidance data to the destination is adapted to be transmitted by compressing it in order to shorten the time required for the data transmission. In this case, at the navigation apparatus that received the compressed data, the compressed data for all over the route is expanded at one time when the data is to be reproduced. Therefore, there is a problem in that a large load is given to the navigation apparatus and therefore a processing performance is lowered.
Further, the expanded route/guidance data must be stored in a storage device until the vehicle on which the navigation apparatus is mounted reaches the destination. In this case, since the data size of the expanded route/guidance data is too large, it is necessary to equip with a large capacity storage device in the navigation apparatus, which results in a problem in that the size of the storage device becomes too large.
Furthermore, when a communication error occurs during the data transmission from the navigation base, it becomes necessary to retransmit all the compressed route/guidance data to the destination again after the communication be restored, thus leading to further increase in the required communication time. Further, normally, the compressed route/guidance data forms single data as a whole. Therefore, in the case where a part of the route/guidance data has been already received before the occurrence of the communication error and then the remaining data has been received later after the restore of the communication, there is a case that it is difficult to process and reproduce the route/guidance data. This means that there is a problem in that restoring process is difficult when communication error occurs during the data transmission, so that it is not possible to provide the route/guidance information in an effective manner.
Moreover, when a communication error such as interruption of communication occurs during the data transmission, it is not possible to carry out route guidance until the communication be restored. This prevents the users from effectively utilize the route guidance, thus leading to, in the worst case, the situation that the user can not start from the present position.
(3) In the conventional communications navigation system described above, all the route/guidance data from the departure point to the destination produced at the navigation base apparatus is transmitted to the navigation apparatus. In this way, at the vehicle, route/guidance information corresponding to the current position of the vehicle is successively displayed in the display of the navigation apparatus.
However, in the case where vehicles return from the destination to the departure point, it is believed that most of the vehicles travel on the same road in the opposite direction not so long as there is a specific situation such as a road repair or the like. In this case, the navigation base searches the same route on the both directions thereof, and substantially the same route/guidance data is transmitted. Further, there is also a case where a part of the newly searched route is common with that of the previously searched route.
In the conventional communications navigation system, however, route/guidance data for a newly searched route is transmitted from the navigation base to the navigation apparatus every time upon a search request. Therefore, if there are common portions between the previously transmitted route/guidance data which is stored in the navigation apparatus and the route/guidance data for the newly searched route, the navigation base is required to transmit unnecessary route/guidance data. This results in unnecessary increase in the communication time and the communication costs.